The British monarchy states that the title of "Princess" only applies to those born into the family, like Princess Charlotte, although Kate Middleton is technically styled as Princess William and is considered a princess by marriage, according to the blog Royal Central.
While Harry's mother, Diana, was styled as Diana, Princess of Wales, the title alone did not make her a princess, despite the public's common reference to her as Princess Diana.
If Harry is not presented with a peerage upon marriage, Markle will become HRH Princess Henry of Wales, although it has been speculated that Harry will be made a duke and Markle a duchess. Before a dukedom can be granted, it must be available, and the pair is expected to be awarded Sussex, according to Royal Historian Marlene Koenig.
The title was last held by Prince Augustus Frederick, who died in 1843. Should that be the case, Harry would become the Duke of Sussex and Markle would become Her Royal Highness Meghan, Duchess of Sussex.
When Prince William married Middleton, he added the Duke of Cambridge to his title and Middleton became Her Royal Highness, Catherine Duchess of Cambridge after the Queen granted the dukedom to the pair.
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